He encouraged both sides in the conflict to work hard to return to the ceasefire brokered in 2012.

President Barack Obama commented on the Israel incursion into Gaza on Friday at the end of a press conference on the Malaysia Airlines MH17 crash in eastern Ukaine. (Jacquelyn Martin/Associated Press)

United Nations Secretary-General Ban Ki-moon is expected to leave Saturday for the Middle East to help mediate the conflict between Israel and Hamas, a UN official said Friday.

Under-Secretary-General for Political Affairs Jeffrey Feltman also said at a UN Security Council emergency meeting on the conflict in Gaza that a ceasefire is "indispensable" for urgently needed humanitarian efforts to succeed. He said the UN refugee agency was already stretched to its limit.

The Palestinian UN ambassador threatened to go to UN and international courts, which would likely include the International Criminal Court, if the Security Council doesn't act to protect Palestinian civilians from Israeli attack and end the conflict in Gaza.

Ground offensive 'not self-defence'

Riyad Mansour said that Israel's "savage" aggression into Gaza "cannot be justified by any means."

"It is not self-defence," he said. "It is a vengeful military aggression intentionally planned and perpetrated by the occupying power on the civilian population under its occupation."

Officials said the offensive could last up to two weeks as the prime minister ordered the military to prepare for a "significantly" wider campaign.

Israel launched the operation late Thursday, following a 10-day campaign of more than 2,000 air strikes against Gaza that had failed to halt relentless Hamas rocket fire on Israeli cities.

Israel's first major ground offensive in Gaza in just over five years came as Egyptian ceasefire efforts stalled. Earlier this week, Israel accepted Cairo's offer to halt hostilities, but Hamas refused, demanding that Israel and Egypt first give guarantees to ease the blockade on Gaza.

A masked Palestinian protester holds a Palestinian flag as he runs away from tear gas fired by Israeli soldiers during a protest against the Israeli offensive in Gaza. (Abed Omar Qusini/Reuters)

Israel had been reticent about launching a ground offensive for fear of endangering its own soldiers and drawing international condemnation over mounting Palestinian civilian deaths.

Israeli leaders have said the aim is to weaken Hamas militarily and have not addressed the possibility of driving the Islamic militants from power. Officially, the goal remains to destroy a network of tunnels militants use to infiltrate Israel and attack civilians.

On Thursday, 13 heavily armed Hamas militants tried to sneak in through such a tunnel, but were stopped by an airstrike after they emerged some 250 metres inside Israel.

Hamas weaker now

Hamas has survived Israeli offensives in the past, including a major three-week ground operation in January 2009 from which it emerged militarily weaker, but in each case it recovered. The groups controls an arsenal of thousands of rockets, some long range and powerful, and it has built a system of underground bunkers.

Netream Netzleam holds the body of her daughter Razel, 1, who medics said died on Friday from injuries sustained in an Israeli air strike on Thursday afternoon, at her funeral in Rafah in the southern Gaza Strip. (Finbarr O'Reilly/Reuters)

But Hamas is weaker now than it was during the previous two offensives — from 2008-09 and 2012 — with little international or even regional support , even among traditional Gaza supporters Turkey and the Gulf-state of Qatar.

French Foreign Minister Laurent Fabius was travelling Friday to Egypt, Jordan and Israel as part of a diplomatic push to stop the fighting in Gaza.

He said in a statement that he wants a ceasefire and lasting truce "that responds to Israel's security needs and Palestinian economic needs."

The operation may be its best opportunity for Israel to rid the strip of the group — which much of the world considers to be a terrorist organization.

But an operation that last weeks could take a heavy toll in both casualties and cost, and most Israelis have no wish to retake Gaza, which the country effectively gave up in 2005.

Egypt supports a ceasefire, but not Hamas or its conditions, which include a lifting to the siege of Gaza and completely open borders into the Sinai — where Egypt is already fighting Islamic extremists.

Israeli defence officials said soldiers faced little resistance during the first night of the ground operation, but the longer the military keeps a presence in Gaza, the greater the risk for heavy casualties on both sides.

Forces are expected to spend a day or two staking ground and are working in the north, east and south of the Gaza Strip. Then, they are expected to move to the second phase, which is to destroy tunnels, an operation that could take up to two weeks.

After thousands of troops had been on standby for several days, Israeli Prime Minister Benjamin Netanyahu said he ordered the military to prepare for a "significant expansion" of the ground offensive.

"Since it is not possible to deal with the tunnels only from the air, our soldiers are doing it also from the ground," he said before a special Cabinet meeting in Tel Aviv.

"We chose to begin this operation after the other options were exhausted and with the understanding that without the operation the price we will pay can be very high."

Throughout the night, the thud of tank shells echoed across Gaza, often just a few seconds apart. Several explosions from Israeli missile strikes shook high-rise buildings in central Gaza City and sent pillars of smoke into the sky.

First Israeli soldier death

The wounded were rushed to Gaza's main Shifa Hospital, including several members of the same family struck by shrapnel from tank shells. Among those hurt were a toddler and a boy of elementary school age, their bodies pocked by small bloody wounds.

Smokes rises from buildings in Gaza City following what witnesses said was an Israeli air strike on Friday. (Suhaib Salem/Reuters)

Gaza health officials said at least 20 Palestinians have been killed since the ground operation began, including three teenage siblings killed by shrapnel from a tank shell attack. At the morgue, one of the victim's faces was blackened by soot and he and his siblings were each wrapped in a white burial shroud.

The Israeli military said it killed 17 militants in different exchanges of fire, while 13 were captured after surrendering. It was not immediately clear if the militants were among the casualties reported by Gaza authorities.

"The ground offensive does not scare us and we pledge to drown the occupation army in Gaza mud," Hamas spokesman Sami Abu Zuhri said in a statement.

The Israeli military said one soldier was killed in the northern Gaza Strip, the first Israeli casualty among troops. The circumstances behind the death of Staff Sgt. Eitan Barak, 20, were not immediately clear, with Hamas' military wing saying it ambushed Israeli units in the northern town of Beit Lahiya and caused casualties but Israeli media saying it was likely a case of friendly fire.

An IDF soldier was killed in action during the ground operation in Gaza last night. His family has been notified.

Tanks, infantry and engineering forces were operating inside the coastal strip. The military said it targeted rocket launchers, tunnels and more than 100 other targets, and that a number of soldiers were wounded throughout the night.

Since the July 8 start of the air campaign, more than 260 Palestinians have been killed and more than 2,000 wounded, Palestinian health officials said. A fifth of the victims were children, according to UNICEF.

In Israel, one civilian died and several were wounded.

Palestinians salvage from the rubble of their destroyed homes in an apartment building that was hit by an Israeli missile strike in Gaza City Friday. (The Associated Press)

Israeli public opinion appears to strongly support the offensive after days of unrelenting rocket fire from Gaza and years of southern Israeli residents living under the threat. Gaza militants have fired more than 1,500 rockets at Israel over the past 11 days.

Prior to the Israeli Cabinet meeting, several ministers said they expected a prolonged offensive.

"We need to go in and finish the job. We need to eliminate every terrorist. They have no immunity." said Uri Ariel, a Cabinet minister from the hard-line Jewish Home party.